It has heretofore been the practice in the construction of vehicular shock absorbers of the type including a reciprocable piston located within a fluid chamber and provided with valve means for controlling the fluid flow through selected fluid ports, to operatively connect the piston assembly to one end of an associated piston rod which projects coaxially outwardly from the shock absorber and is connected, either directly or indirectly, to a portion of a vehicle suspension. Various means have been utilized in the prior art for operatively securing the piston assembly to the piston rod, one of the most common of which consists of providing some type of a threaded element, such as a nut or the like, and threadably connecting said element to an externally threaded portion formed on the adjacent end of the associated piston rod. One such example of this prior art construction is shown in U.S. Pat. No. 3,757,910, dated Sept. 11, 1973, and assigned to the assignee of this application. The aforesaid procedure of operatively connecting the piston assembly to the piston rod by means of a threaded element or nut has been found to be objectionable for a number of reasons, not the least of which is the cost and time required in machining the nut and associated piston rod.
Various solutions have been proposed to such prior art designs, including the idea of connecting the piston assembly to the associated piston rod by means of a welding operation, such as shown in U.S. Pat. No. 3,724,615, issued Apr. 3, 1973, wherein the piston body is connected by means of a weld operation to the adjacent end of the associated piston. This method of assembly, however, has not been entirely satisfactory due to the fact that it requires extremely costly equipment and proper quality control can be difficult to maintain. One more serious problem results from the aforesaid type of construction and that resides in the frequent necessity of providing some type of a sliding seal around the outer periphery of the piston body to assure that a fluid tight joint is provided between the outer periphery of the piston and the inner periphery of the adjacent cylinder wall, whereby to assure against fluid by-passing the piston instead of flowing through the valve ports normally provided therein. Such ancillary fluid sealing means have been required, at least in part, due to the eccentricity or "radial runout" between the axis of the piston rod and piston body which, in many instances, may be in the magnitude of several thousands of an inch. Moreover, while the aforesaid fluid seals have been found to be at least a satisfactory solution to such eccentricity between the piston and piston rod, in certain circumstances, the seals themselves become irregularly worn, thus permitting fluid by-passing and premature failure of the shock absorber.
A related short-coming of certain prior art designs, such as that shown in U.S. Pat. No. 3,187,847, results when the piston assembly is connected to the associated piston rod by means of a cold or hot "upsetting" process which results in substantial molecular distortion or alteration of the piston rod which may, under certain circumstances, have an adverse effect on the structural integrity of the material interconnecting the piston and piston rod, with the result that the connection falls short of the requisite tensile strength specifications in the industry.
The present invention is directed toward a new and improved shock absorber construction and method and apparatus for assembling the same which overcomes the various shortcomings of prior art designs. In particular, the present invention is directed toward a new piston-piston rod assembly wherein the piston rod is operatively connected to the piston assembly in a manner such that an extremely strong joint is provided therebetween without any sacrifice in the desired concentricity between the piston body and piston rod. The connection is achieved by means of inserting a tool into the valve chamber of the piston from the opposite side thereof from which the piston rod extends and operatively moving the tool in a preselected manner. Specifically, the tool is moved in a generally cycloidal path such that an overlapping rosette type pattern is formed. By so moving the tool, small portions of the material from which the piston rod is fabricated is moved radially outwardly to form an enlarged diameter head, with the material moving so gradually that there is virtually no molecular alteration destruction of the piston rod material. Accordingly, the unit strength of the resultant joint is extremely high. One particular advantage of the invention resides in the fact that the tool may be inserted interiorly of the valve chamber to a position where the tool may contact the end of the piston rod without in any way interferring with the fixture of apparatus which functions to coaxially support the piston and piston rod during assembly thereof. Accordingly, the highest concentricity standards may be readily achieved, as compared to prior art designs. An additional feature of the present invention resides in the fact that by properly designing the assembly tooling or fixtures, the piston and piston rod will be self-centering or aligning during the fastening or securing operation, thus assuring concentricity of the resultant assembly. Accordingly, the present invention provides a piston-piston rod assembly which is of substantially higher quality than prior art designs and yet may be manufactured at a considerable savings in capital equipment.